2018ApJ...860..161S

We report the first results of AS2UDS, an 870 µm continuum survey with the Atacama Large Millimeter/Submillimeter Array (ALMA) of a total area of ∼50 arcmin² comprising a complete sample of 716 submillimeter sources drawn from the SCUBA-2 Cosmology Legacy Survey (S2CLS) map of the UKIDSS/UDS field. The S2CLS parent sample covers a 0.96 degree² field at σ₈₅₀ = 0.90 ± 0.05 mJy beam^–1. Our deep, high-resolution ALMA observations with σ₈₇₀ ∼ 0.25 mJy and a 0.''15-0.''30 FWHM synthesized beam, provide precise locations for 695 submillimeter galaxies (SMGs) responsible for the submillimeter emission corresponding to 606 sources in the low-resolution, single-dish map. We measure the number counts of SMGs brighter than S₈₇₀ >= 4 mJy, free from the effects of blending and show that the normalization of the counts falls by 28% ± 2% in comparison with the SCUBA-2 published counts, but that the shape remains unchanged. We determine that 44_–14⁺¹⁶% of the brighter single-dish sources with S₈₅₀ >= 9 mJy consist of a blend of two or more ALMA-detectable SMGs brighter than S₈₇₀ ∼ 1 mJy (corresponding to a galaxy with a total-infrared luminosity of L_IR >= 10¹² L_☉), in comparison with 28% ± 2% for the single-dish sources at S₈₅₀ >= 5 mJy. Using the 46 single-dish submillimeter sources that contain two or more ALMA-detected SMGs with photometric redshifts, we show that there is a significant statistical excess of pairs of SMGs with similar redshifts (<1% probability of occurring by chance), suggesting that at least 30% of these blends arise from physically associated pairs of SMGs.